Synaptic plasticity in many different forms is widely recognized as essential to normal brain development, to homeostasis of the mature brain, and to our abilities to adapt to changing environments and injury and to learn. There is relatively little information, however, to link particular known forms of synaptic plasticity to particular forms or sites of behaviorally relevant brain circuit plasticity. This gap in our knowledge prevents us from efficiently pinpointing synapses of known functional relevance as we explore the molecular, synaptic, and circuit bases of memory and its disorders. The experiments proposed here will exploit unique advantages of the mouse whisker sensory system to explore basic mechanisms of neocortical synaptic plasticity. The work will apply a powerful new high-resolution proteomic imaging method called array tomography (AT) to measure molecular and structural characteristics of cortical synapse populations at the level of individual synapses. AT has unique abilities to resolve individual synapses in native circuit tissue context, to measure dozens of distinctive molecular markers (e.g., diverse receptor, transporter, signaling, scaffolding and adhesion proteins) at each synapse, and to do so with very high experimental throughput. Thus, AT can determine a high dimensional molecular signature for each individual synapse in very large populations and differentiate specific synapse subpopulations on the basis of such molecular signatures. The proposed research will develop and apply a novel AT-based screening strategy to search in an unbiased fashion for patterns of structural and molecular change occurring in specific mouse neocortical synapses in reaction to specific sensory adaptation and associative conditioning procedures.